The present invention relates to hot-dip galvanizing. More particularly, the invention relates to a galvanizing bath which produces a uniform alloy layer, and a galvanized iron article produced using the galvanizing bath.
Hot-dip galvanizing has been widely applied to iron and steel materials because an alloy layer of Zn and Fe contained in an iron article is formed to exhibit excellent adhesion, and excellent corrosion resistance is provided due to a sacrificial anode effect.
A galvanized coating includes an FeZn7 (Fe: 7 to 11%) hexagonal δ1 alloy layer formed on the underlying iron, an FeZn13 (Fe: about 6%) ζ (zeta) alloy layer formed on the δ1 alloy layer and having a columnar structure belonging to a monoclinic system, and an η zinc layer formed on the ζ alloy layer and having a dense hexagonal structure.
The ζ alloy layer in such a galvanized coating structure is important for increasing the thickness of the galvanized coating. On the other hand, since the ζ alloy layer has a columnar structure, the ζ alloy layer has a low degree of symmetry as compared with other layers. If the thickness of the ζ alloy layer is nonuniform, corrosion resistance may decrease, or the galvanized coating may become fragile.
Moreover, since the ζ alloy layer is whitish as compared with the zinc layer, the appearance of the coating is impaired when the ζ alloy layer is partially formed near the surface of the galvanized coating.
JP-A-2004-285387 discloses technology of adding Al to a bath in an amount of 0.10 to 0.6% in order to improve the appearance of the galvanized coating. This technology aims at forming a Zn—Al—Fe ternary alloy layer.
JP-A-4-154950 discloses an Fe—Zn alloy galvanizing bath containing 0.1 to 10% of Fe.
JP-A-63-247331 discloses a colored hot-dip galvanizing zinc alloy containing 0.2 to 0.7% of Ti.
JP-A-63-247332 discloses an iridescent hot-dip galvanizing zinc alloy containing 0.1 to 0.8% of Mn.